**1. Introduction**

Intense and sustained research into single crystals and polycrystalline assemblies continues to produce advanced materials crucial to the function of modern and future electronic and photonic devices. Device function is not only based upon the optical and electronic properties of the active material (for example, the light absorbing layer in molecular solar cells), but also the molecular and supramolecular ordering. Herein we will focus on devices and technologies where single crystalline materials are employed, and where appropriate, discuss competing materials and devices where polycrystalline or amorphous materials are used. We acknowledge that there is a sustained push to use solution processable conjugated polymer films for several similar technological applications; however, these films are typically disordered and suffer from impurities and chemical defects inherent to synthetic macromolecules making definitive characterization using X-ray diffraction difficult or impossible. Small molecule crystalline materials, on the other hand, can be obtained in high purity with a high level of organization in the solid state.

Additionally, our discussion will be largely limited to organic materials as there is a drive to replace conventional inorganic active materials (for example the semiconductor channel layers in transistors) with organic compounds. The reasoning for this lies in the fact that organic compounds can be synthesized and purified relatively easily using bench-top techniques without the requirement of high temperatures and energy intensive processes. As an example, to obtain electronics and solar cell grade silicon several high temperature steps are required to reach the necessary purities in excess of 99.9999%.1 Organic compounds can be highly purified using conventional solution crystallization or vapor growth2 methods yielding materials which exhibit impressive performance, in some cases rivaling their inorganic counterparts.3

For this review we selected to focus on two areas: (i) photochromic molecules that can be used for molecular scale data storage and as actuators capable of turning molecular level changes into macroscopic motion, (ii) organic field effect transistors and organic photovoltaics that

© 2012 Patel and Benedict, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Patel and Benedict, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

make use of single crystals for charge transport. Other important areas of research such as catalysis, nonlinear optics, magnetic materials, and biology also make effective use of crystalline materials but are beyond the scope of this review.
